In a lot of processes it is necessary to weigh the items being processed as they are being processed which means that it is necessary to continuously weigh products, objects or items as they pass for example on a conveyor.
In other processes it is necessary or desirable to weigh the items batch-wise or as singles or in a stepwise manner, for these purposes the weighing apparatus' may be used as stand-alone units.
In the art numerous devices are available. It is, however, important to select a suitable weighing device depending on the speed with which the items to be weighed passes the weighing installation and also the weight of the objects which are to be weighed and furthermore which weighing tolerances can be accepted during the weighing procedure. A proven constructional principle in this connection is to connect the part of the weighing device which is influenced by the weight of objects passing for example on a conveyor to the base or non-movable part by a parallelogram. The parallelogram provides a very stiff construction yet a construction which is suitable to in very controlled manner move slightly but consistently and potentially linearly under the influence of loads.
Examples of devices incorporating a parallelogram in the construction of the weighing cell are for example disclosed in EP 1564534 and EP 3021094. Common for both these devices and for many other devices in the art is the fact that in order to obtain the necessary precise weighing results the components from which the apparatus is made needs to be made very precisely. Particularly in the prior art it is common routine to machine the part of the weighing device incorporating the parallelogram in order to obtain a substantially 100% parallel construction such that as the movable part moves relatively to the base part it is ensured that this movement will occur in a perfect parallelogrammic movement.
One such example is mentioned as prior art in U.S. Pat. No. 4,662,464, where a parallelogrammic weighing system is described. The system comprises two parallelograms each having a separate (vertical) post (3a, 3b). Each vertical post is connected in either end by (horizontal) leaf springs to a vertical link. The vertical link is common for both parallelograms. The vertical link is a machined monolith, providing fastening for the four ends of the leaf springs, and (approximate) equal spacing both for the adjacent leaf springs and for the upper and lower leaf springs.
Equal spacing and thereby perfect parallelograms are a pre-requisite for precise weight measurements. A disadvantage of this construction is the many parts from which it is assembled. This problem is addressed in the invention of U.S. Pat. No. 4,662,464 by providing a machined monolith structure in both ends of the flexible (horizontal) elements. Although by adequate and expensive machining this construction may overcome the shift-problems described and other inaccuracies for these types of weighing devices, the costs involved in achieving the desired accuracies (very low tolerances) is extremely high.